Aedes aegypti mosquitoes are the primary vector for dengue, yellow fever and chikungunya. The vector is present in most urban communities stretching along the U.S.-Mexico border yet dengue transmission has only been noted in two U.S. border communities in Texas, notably Brownsville and Laredo, TX. While many argue that this is a result of different social factors across the border, this fails to explain the lack of transmission in some communities in northern Mexico, notably Nogales, Sonora. Nogales is a large urban area on the Mexican side of the border with large vector populations and a constant influx of people from dengue endemic areas yet no local cases have been recorded. Vector presence alone is not sufficient to cause disease transmission;vectors must also survive long enough to blood feed and become infected, have the parasite develop and feed again, transmitting the virus. Climatic influences on the lifespan and behavior of Ae. aegypti may also influence the risk of dengue transmission in this region which lies at the boundary of both virus and vector. Geographic areas such as this, at the edge of the range of disease and vector, are at greatest risk of emergence and reemergence of infectious diseases. We will assess the relative age structure of Ae. aegypti populations in nine cities with varying climates in the southern United States and Sonora, MX by trapping and molecularly determining the ages of adult Ae. aegypti during four mosquito seasons. We expect that cities with extremely dry and hot conditions will have younger Ae. aegypti populations. Cities with reported dengue transmission we expect to have Ae. aegypti that are on average younger when taking their first blood meal than cities without transmission. To determine how the age of Ae. aegypti population vary over a finer spatial scale, we will sample adults in 150 households in Hermosillo, Mexico;a city with a history of dengue outbreaks. Additionally, we will survey the households for potential social and environmental factors that mediate the relationship between climate and longevity. We expect to find significant variability across households. Residences with older mosquitoes will have more vegetation, fewer barriers to mosquito access and indoor resting sites, no competing vector species and no control measures. From the information gathered in the first two objectives, we will construct a model to predict the likelihood of the expansion of Ae. Aegypti populations that survive long enough to transmit dengue and thus changes in risk of dengue transmission. We expect that models of risk of dengue under climate change scenarios will decrease in areas that surpass a heat and dryness threshold and will increase in populations with increasing moisture.